No structural information is available, but Pa0,I is reported to possess hexagonal symmetry (Table 111). Protactinium tetraiodide is a dark green solid which can be prepared by reducing the pentaiodide with aluminum a t about 400°C (44). The structure is not known. On sublimation in silica above 500°C small amounts of the pink oxydiiodide, PaOI,, are formed. Structural information is not available for the oxydiiodide, but it is known to be isomorphous with ThOI,. Despite the fact that excess aluminum failed to reduce protactinium tetraiodide it was recently reported (123)that heating the pentaiodide in a continuously pumping vacuum a t 35OoC for 1 week resulted in the elimination of iodine and the black residue was identified crystallographically as PaI,.

The structure of this compound is currently being determined. ) have indicated that the product contains a variable amount of iodate (Pa:IO, between 1 : 0 . 0) and more than twelve molecules of water. The infrared vibration associated with the iodate was observed as a broad band between 600 and 870 cm-'. The white solid slowly decomposes with the liberation of iodine. The pentavalent phosphate, which precipitates from sulfuric acid solution on the addition of orthophosphoric acid is reported to have the composition PaO(OH)PO,.